Lithography system and method

ABSTRACT

A lithography system has a projection lens that includes a first optical element and a first sensor subframe. The projection lens also includes first sensor which is configured to detect a position of the first optical element with respect to the first sensor subframe. The projection lens further includes a second sensor which is configured to detect a position of a wafer with respect to the first sensor subframe.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation of, and claims benefit under35 USC 120 to, international application PCT/EP2017/078490, filed Nov.7, 2017, which claims benefit under 35 USC 119 of German Application No.10 2016 225 707.2, filed Dec. 21, 2016. The entire disclosure of theseapplications are incorporated by reference herein.

FIELD

The present disclosure relates to a lithography system and a method forproducing a projection lens.

BACKGROUND

Microlithography is used for producing microstructured components, forexample integrated circuits. The microlithographic process is carriedout with a lithography apparatus, which has an illumination system and aprojection system. The image of a mask (reticle) illuminated via theillumination system is in this case projected via the projection systemonto a substrate (for example a silicon wafer) coated with alight-sensitive layer (photoresist) and arranged in the image plane ofthe projection system, in order to transfer the mask structure to thelight-sensitive coating of the substrate.

Driven by the desire for ever smaller structures in the production ofintegrated circuits, currently under development are EUV lithographyapparatuses that use light with a wavelength in the range of 0.1 nm to30 nm, in particular 13.5 nm. In the case of such EUV lithographyapparatuses, because of the high absorption of light of this wavelengthby most materials, reflective optical units, that is to say mirrors,have to be used instead of—as previously—refractive optical units, thatis to say lens elements.

The positions of the mirrors which are arranged in a projection systemfor defining a beam path can be detected with the aid of sensors. Inthis case, the sensors are mounted for example on a mirror or on asensor frame, wherein the position of the mirror relative to the sensorframe can be detected. A sensor frame can be divided into a plurality ofoscillation-decoupled sensor subframes. By virtue of a plurality ofsensor subframes being provided, the sensor subframes can haveintrinsically smaller dimensions or a reduced mass. This reduces thequasi-static deformation as a result of low-frequency dynamic excitationat the system and thus an image oscillation. If a plurality of sensorsubframes are present which are referenced to one another, acorresponding measurement complexity arises, which results inmeasurement inaccuracies. Furthermore, alignment inaccuracies betweenthe sensor subframes can occur.

WO 2013/178277 A1 discloses a projection system having two sensorsubframes, to which in each case three optical elements are referencedwith the aid of sensors. Furthermore, the sensor subframes haveadditional sensors in order to detect position data between the sensorsubframes.

SUMMARY

The present disclosure seeks to provide an improved lithography systemand an improved method for producing a projection lens.

Accordingly, a lithography system is proposed. The lithography systemincludes a projection lens. The projection lens includes a first opticalelement, a first sensor subframe, a first sensor, which is configured todetect a position of the first optical element with respect to the firstsensor subframe, and a second sensor, which is configured to detect aposition of a wafer with respect to the first sensor subframe.

By virtue of detecting the position of the first optical element and theposition of the wafer with respect to a single sensor subframe, it ispossible to avoid a measurement error that would occur between twosensor subframes referenced to one another. For the case where theprojection lens includes only the first sensor subframe and no furthersensor subframe (that is to say no divided sensor frame), the firstsensor subframe can also be referred to as “one”, if appropriate also “asingle” sensor frame. The sensor frame is integral, for example. Thishas the advantage that no sensor subframes need be referenced to oneanother, such that the metrology associated with this is obviated.Furthermore, measurement and alignment errors that occur whenreferencing a plurality of sensor subframes to one another areeliminated. Preferably, the projection lens includes a multiplicity ofoptical elements and a multiplicity of sensors configured to detectpositions of the multiplicity of optical elements related to that firstsensor subframe. By way of example, the projection lens includes atleast or exactly 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 or 14 opticalelements that are referenced to the first sensor subframe.

In accordance with one embodiment, the lithography system includes thewafer. Furthermore, the projection lens includes the first opticalelement, a second optical element, the first sensor subframe and asecond sensor subframe, which are oscillation-decoupled from oneanother, a third sensor, which is configured to detect a position of thefirst optical element with respect to the second sensor subframe.

Position sensors in lithography systems are blind in such a range inwhich the difference between actual and setpoint positions of themonitored optical element, wafer and/or reticle does not exceed aspecific threshold value (in the present case also referred to asmeasurement error). Furthermore, the measurement error is influenced bymeasurement noise. Such measurement errors can add up, depending on thedesign of the measurement and of the lithography system. This canultimately have the consequence of erroneous guiding of the monitoredoptical element, wafer and/or reticle via a corresponding actuator.

What is advantageous in accordance with this embodiment is thatincorrect positionings of the monitored optical elements, of the waferand/or of the reticle (also referred to jointly as elements) in relationto the image effect jointly achieved by these elements mutuallycompensate for one another. Effects of such incorrect positionings aredescribed with the aid of the so-called line-of-sight sensitivity(so-called LoS sensitivity; definition below).

This insight can be used to the effect that the measurement errors ofthe position sensors are accorded a lesser significance because theincorrect positionings of the monitored optical elements, of the waferand/or of the reticle have a lesser effect in the imaging. To that end,the measurement is designed such that the LoS sensitivities within agroup of elements which is assigned to a sensor subframe as measurementreference compensate for one another as much as possible. This isachieved in particular by virtue of the fact that the third sensordetects the position of the first optical element with respect to thesecond sensor subframe, and the first sensor detects the position of thefirst optical element with respect to the first sensor subframe, and thesecond sensor likewise detects the position of the wafer with respect tothe first sensor subframe.

Advantageously, the lithography system includes an EUV lithographyapparatus, in which a wavelength of the working light is in particularbetween 0.1 and 30 nm, or a DUV lithography apparatus, in which thewavelength of the working light is in particular between 30 and 250 nm,with the projection lens.

The LoS sensitivity for shifts is defined as follows: LoS=(s μm)/(1 μm).Furthermore, the LoS sensitivity for tilts is defined as follows: LoS=(sμm)/(1 μrad). If an optical element, a reticle or a wafer is shifted by1 μm or tilted by 1 μrad, then the image shifts by s μm. Preferably, anLoS sensitivity is determined for a constant numerical aperture.

A “position” of one element with respect to another element means in thepresent case that position data between the two elements are detectedwhich allow a conclusion to be drawn about the location of the twoelements with respect to one another in up to 6 degrees of freedom. Inother words, it is possible to draw a conclusion about distances betweenthe two elements in up to three spatial directions, which are forexample perpendicular to one another, and about tilts of the twoelements with respect to one another by up to three rotation axes.

An “optical element” is for example a mirror, a lens element, an opticalgrating or a lambda plate.

A “sensor frame” means in the present case a structure with respect towhich position data of one or more elements, which can be embodied asoptical elements, reticles and/or wafers, are determined. The sensorframe can be divided into a plurality of sensor subframes that areoscillation-decoupled from one another. Sensors or parts of the sensorscan be connected to the sensor subframe. “Frame” in the present casedoes not necessarily presuppose a frame-shaped structure, but ratheralso encompasses for example a platform or a plate. The sensor subframesare embodied in a rigid fashion. In this case, a sensor subframe caninclude one or more of the following materials: silicon carbide (SiC),reaction-bonded silicon-infiltrated silicon carbide (SiSiC), cordierite,aluminium oxide (Al₂O₃), aluminium nitride (AlN), glass ceramics such asZerodur, titanium silicate glass (also known as ULE') etcetera, steel,aluminium or other metals and alloys. More than two sensor subframes,for example a third and/or a fourth sensor subframe, can be provided.

“Oscillation decoupling” should be understood to mean that virtually noforces, vibrations and/or oscillations are transmitted from one sensorsubframe to another sensor subframe, or vice versa. This can be achievedfor example via a very soft and/or resilient mounting of the sensorsubframes.

A “sensor” in the present case can also include a plurality ofsubsensors (e.g. one subsensor for each spatial direction) that detectdata separately from one another, with the aid of which it is possibleto draw conclusions about the position of one element with respect toanother element. The sensor is embodied for example as a capacitive oroptical sensor. The sensor can include a transmitter and a receiver,which can be realized in one unit. Consequently, the sensor can bemounted only on a first element (“element” here denotes optical element,wafer or reticle and/or sensor subframe). The second (monitored) elementthen itself serves as a measurement object (e.g. reflector of light).Alternatively, the transmitter is mounted on a first element and thereceiver is mounted on a second element, wherein a position of thesecond element with respect to the first element is detected. In thiscase, the receiver is the measurement object.

In accordance with a further embodiment, the first and second opticalelements at least partly define a beam path of the projection lens. Thefirst optical element is arranged as the last optical element in thebeam path upstream of the wafer.

This has the advantage that the position of the last optical element andthe position of the wafer are detected with respect to a single sensorsubframe, such that the LoS sensitivity of the wafer and of the lastoptical element with respect to remaining beam path defining elements isreduced. Preferably, the projection lens includes 4 to 12 opticalelements. “Beam path” in the present case is understood to mean thegeometric course of light rays towards a target object, for example awafer to be exposed. Furthermore, “working light” is understood to meanthe light that radiates from a light source indirectly or directlytowards an optical element and serves for an exposure of the wafer. Thewavelength of the working light is in particular between 0.1 and 30 or30 and 250 nm.

In accordance with a further embodiment, the projection lens includes athird optical element, which further defines the beam path, and a fourthsensor, which is configured to detect a position of the third opticalelement with respect to the first sensor subframe. The third opticalelement is arranged as the penultimate optical element in the beam path.

Consequently, the position of the last optical element, the position ofthe penultimate optical element and the position of the wafer aredetected with respect to a single sensor subframe. A group of elementsis thus referenced to a single sensor subframe. It has been found,surprisingly, that in the case of such an arrangement the LoSsensitivity of this group (including wafer, penultimate optical elementand last optical element) with respect to remaining beam path definingelements is low.

In accordance with a further embodiment, the lithography system includesa reticle, wherein the projection lens includes a fifth sensor, which isconfigured to detect a position of the reticle with respect to thesecond sensor subframe.

The position of the reticle and the position of the second opticalelement are thus detected with respect to the same sensor subframe.Preferably, the second optical element is the element arranged at thefirst location in the beam path in the projection system, such that theworking light from the reticle is incident directly on the secondoptical element. Preferably, the projection lens includes a furtheroptical element and a further sensor, which is configured to detect aposition of the further optical element, arranged in particular at thesecond location in the beam path. This has the advantage that a group ofelements (reticle, first optical element and, if appropriate, thefurther optical element) are referenced to the second sensor subframeand the LoS sensitivity of this group with respect to the remaining beampath defining elements is low.

In accordance with a further embodiment, the projection lens furthermoreincludes a sixth sensor, which is configured to detect a position of thesecond sensor subframe with respect to the first sensor subframe.

This has the advantage that position data between the first and secondsensor subframes, which are oscillation-decoupled from one another, canbe determined at any time. Thus, the position of the first opticalelement with respect to the second optical element can also be detectedindirectly by determined data of the third sensor, of the first sensorand of the sixth sensor being combined/computed with one another. By wayof example, a measurement error that occurs as a result of a measurementof the sixth sensor leads to an incorrect positioning of the secondoptical element and of the reticle in an identical direction and by anidentical absolute value. However, since a sum of an LoS sensitivity ofthe second optical element and an LoS sensitivity of the reticle issmall, the incorrect positionings have a small influence on the imageeffect, such that this measurement error slightly increases an imagevibration. “Image vibration” in the present case means for example thatthe image experiences a slight, in particular undesired, movement.

In accordance with a further embodiment, the projection lens furthermoreincludes a carrying frame, a first actuator, which is configured toposition the second optical element with respect to the carrying frame,a second actuator, which is configured to position the first opticalelement with respect to the carrying frame, and a control device, whichis configured to drive the first actuator for positioning the secondoptical element depending on the position detected by the third sensor,the position detected by the first sensor and the position detected bythe sixth sensor, and to drive the second actuator for positioning thefirst optical element depending on the position detected by the firstsensor.

The second optical element is thus aligned depending on the position ofthe first optical element. By way of example, in this case, the secondactuator can have as a manipulated variable merely a setpoint positionof the first optical element, which is preferably independent of thepositions of the remaining optical elements. Consequently, the secondoptical element follows the position of the first optical element. Thisis particularly advantageous for the case where the first opticalelement is arranged at the last location in the beam path.

“Carrying frame” in the present case means a carrying structure to whichat least the optical elements and the sensor subframes are linked. Inthis case, actuators are interposed between the carrying frame and theoptical elements in order to perform targeted relative movements of theoptical elements relative to the carrying frame. The sensor subframesare preferably oscillation-decoupled with respect to the carrying frame.This can be realized for example with the aid of a spring element and/ora damper element, a soft spring element and/or a soft damper elementpreferably being used. In particular, the carrying frame encloses avolume in which the sensor subframes are partly or completely arranged.“Partly arranged within the carrying frame” means that the carryingframe defines an enveloping surface, in particular an at least partlycylindrical enveloping surface, from which a sensor subframe sectionprojects. The carrying frame can include or completely consist of aceramic material or a metallic material. In particular, the carryingframe can be produced from or include a non-oxide ceramic, for example asilicon carbide.

The actuators are embodied for example as Lorenz actuators,piezoactuators or actuators with stepper motors.

In accordance with a further embodiment, the first, second and thirdoptical elements are embodied as mirrors and/or lens elements.

In accordance with a further embodiment, the projection lens includes amirror configuration. The mirror configuration includes six normalincidence mirrors or four normal incidence mirrors and four grazingincidence mirrors or three normal incidence mirrors and seven grazingincidence mirrors. The first, second and third optical elements areselected from the mirrors of the mirror configuration.

Consequently, the mirror configuration can for example include orconsist of six normal incidence mirrors. Furthermore, the mirrorconfiguration can for example include or consist of eight mirrors,namely four normal incidence mirrors and four grazing incidence mirrors.Furthermore, the mirror configuration can for example include or consistof ten mirrors, namely three normal incidence mirrors and seven grazingincidence mirrors.

“Normal incidence mirror” in the present case means a mirror which isarranged in the beam path in such a way that light rays of the workinglight that are incident on a reflective surface of the mirror have anangle of incidence of for example greater than 30°.

“Grazing incidence mirror” in the present case means a mirror that isarranged in the beam path in such a way that light rays of the workinglight that are incident on a reflective surface of the mirror have anangle of incidence of for example less than 30°. Consequently, lightrays impinge with grazing incidence on the reflective surface of themirror.

In accordance with a further embodiment, the first optical element hasan LoS sensitivity and the wafer has an LoS sensitivity. The LoSsensitivity of the first optical element has a different sign from thatof the LoS sensitivity of the wafer.

By virtue of the fact that the signs of the LoS sensitivities within agroup of elements whose positions are detected with respect to the samesensor subframe are different, the LoS sensitivities can at least partlycompensate for one another, such that the LoS sensitivity of this groupwith respect to remaining beam path defining optical elements isreduced. By way of example, an LoS sensitivity of the reticle has adifferent sign and a similar absolute value compared with an LoSsensitivity of the second optical element, such that these LoSsensitivities substantially compensate for one another. Consequently,the measurement error that occurs during measurements of the sixthsensor does not lead to increased image vibration.

In accordance with a further embodiment, a sum formed from the LoSsensitivity of the first optical element, of the wafer and/or of thethird optical element has an absolute value of less than 0.5, less than0.2, less than 0.1 or less than 0.01.

These absolute values preferably apply to a shift of the first opticalelement, of the wafer and/or of the third optical element by 1 μm. Theimage oscillation of the projection lens is thus reduced.

Furthermore, a method for producing a lithography system, in particularas described above, is proposed. The method includes the followingsteps:

-   -   a) arranging a first and second optical element for defining a        beam path,    -   b) providing a wafer at the end of the beam path and/or a third        optical element for further defining the beam path,    -   c) determining the LoS sensitivities of the first and second        optical elements and also of the wafer and/or of the third        optical element,    -   d) assigning the LoS sensitivities to a first and second sensor        subframe in accordance with a first of N combination        possibilities, p1 e) summing the LoS sensitivities assigned to a        respective sensor subframe, provided that not just one        individual LoS sensitivity is assigned,    -   f) storing the sums and individual LoS sensitivities,    -   g) repeating steps d)-f) up to and including the Nth combination        possibility,    -   h) selecting a combination possibility from the N combination        possibilities depending on the stored sums and individual LoS        sensitivities,    -   i) providing sensors configured to detect a respective position        of the first and second optical element and also of the wafer        and/or third optical element with respect to a reference, and    -   j) selecting the first or second sensor subframe as reference        depending on the selected combination possibility.

By virtue of a combination possibility being chosen for which the sumsof the LoS sensitivities are small, incorrect positions of the opticalelements, of the reticle and/or of the wafer on account of measurementerrors of the sensors, in particular of those which detect a position ofone sensor subframe relative to another sensor subframe, in the imageeffect can at least partly cancel one another out, such that an imagevibration is reduced.

The lithography system is a projection lens, for example. Preferably, instep i) a sensor is respectively provided which is configured to detecta position of the second sensor subframe with respect to the firstsensor subframe and/or a position of the second sensor subframe withrespect to a third sensor subframe and/or a position of the first sensorsubframe with respect to the third sensor subframe. If further sensorsubframes are present, sensors are correspondingly provided, such that aposition of each sensor subframe is detected at least with respect to afurther sensor subframe.

In accordance with a further embodiment, the LoS sensitivitiesdetermined in step c) have different signs.

In accordance with a further embodiment, in step h) absolute values ofthe sums and individual LoS sensitivities of a respective combinationpossibility are formed and summed in order to form a total sum.

Consequently, it is possible to determine a total LoS sensitivity forthe projection lens in order to provide a comparability between the Ncombination possibilities.

In accordance with a further embodiment, the absolute values formed areweighted with a respective factor before the summing step.

Additional parameters can thus be introduced in order to additionallyweight the LoS sensitivity of the groups formed. By way of example,design- or system-dictated boundary conditions can be included with theaid of the parameters.

In accordance with a further embodiment, in step h) that combinationpossibility which has the smallest total sum is selected from the Ncombination possibilities.

Consequently, it is possible to select an optimum solution from an LoSsensitivity consideration.

The embodiments and features described for the lithography system applycorrespondingly to the proposed method, and vice versa.

The order of the method steps does not necessarily correspond to theorder of the (alphabetic) enumeration. Rather, the order of the methodsteps can be chosen arbitrarily within the scope of the knowledge of aperson skilled in the art. By way of example, a plurality of methodsteps can be performed in parallel.

The numerals “first, second, third, etc.”, as used for example for thesensors, optical elements, sensor subframes or actuators, serve merelyfor better distinguishability of the respective elements and areinterchangeable as desired. By way of example, the fifth sensor couldalso be designated as fourth sensor. That is to say that in embodimentsdescribed in the present case which include for example only the first,second, third and fifth sensors, the fifth sensor can be designated asfourth sensor.

“A(n)” in the present case should not be understood as restrictive toexactly one element. Rather, a plurality of elements, for example two,three or more, can also be provided. Any other numeral used here, too,should not be understood to the effect that a restriction to exactly thecorresponding number of elements must be realized. Rather, numericaldeviations upwards and downwards are possible.

Further possible implementations of the disclosure also include notexplicitly mentioned combinations of features or embodiments that aredescribed above or below with respect to the exemplary embodiments. Inthis respect, a person skilled in the art will also add individualaspects to the respective basic form of the disclosure as improvementsor additions.

BRIEF DESCRIPTION OF THE DRAWINGS

Further configurations and aspects of the disclosure are the subject ofthe dependent claims and also of the exemplary embodiments of thedisclosure described below. In the text that follows, the disclosure isexplained in more detail on the basis of preferred embodiments withreference to the accompanying figures, in which:

FIG. 1A shows a view of an EUV lithography apparatus;

FIG. 1B shows a view of a DUV lithography apparatus;

FIG. 2A shows a first embodiment of a lithography system;

FIG. 2B shows a second embodiment of a lithography system;

FIG. 3 shows a third embodiment of the lithography system;

FIG. 4 shows a fourth embodiment of the lithography system;

FIG. 5 schematically shows a position determination for opticalelements; and

FIG. 6 shows a block diagram of a method for producing the lithographysystem.

EXEMPLARY EMBODIMENTS OF THE DISCLOSURE

Identical elements or elements having an identical function have beenprovided with the same reference signs in the figures, unless indicatedto the contrary. In so far as a reference sign has a plurality ofreference lines in the present case, this means that the correspondingelement is present multiply. Reference sign lines pointing to concealeddetails are illustrated in a dashed manner. It should also be noted thatthe illustrations in the figures are not necessarily to scale.

FIG. 1A shows a schematic view of an EUV lithography apparatus 100A,which includes a beam-shaping and illumination system 102 and aprojection system 104. EUV stands for “extreme ultraviolet” and refersto a wavelength of the working light of between 0.1 and 30 nm. Thebeam-shaping and illumination system 102 and the projection system 104are respectively provided in a vacuum housing (not shown), the vacuumhousing being evacuated with the aid of an evacuation device (notshown). The vacuum housings are surrounded by a machine room (notshown), in which the drive appliances for mechanically moving oradjusting the optical elements are provided. Moreover, electricalcontrollers and the like can also be provided in this machine room.

The EUV lithography apparatus 100A includes an EUV light source 106A. Aplasma source (or a synchrotron), which emits radiation 108A in the EUVrange (extreme ultraviolet range), that is to say for example in thewavelength range of 5 nm to 20 nm, can for example be provided as theEUV light source 106A. In the beam-shaping and illumination system 102,the EUV radiation 108A is focused and the desired operating wavelengthis filtered out from the EUV radiation 108A. The EUV radiation 108Agenerated by the EUV light source 106A has a relatively lowtransmissivity through air, for which reason the beam-guiding spaces inthe beam-shaping and illumination system 102 and in the projectionsystem 104 are evacuated.

The beam-shaping and illumination system 102 illustrated in FIG. 1A hasfive mirrors 110, 112, 114, 116, 118. After passing through the beamshaping and illumination system 102, the EUV radiation 108A is directedonto the photomask (reticle) 120. The photo-mask 120 can also bedesignated as reticle. The photomask 120 is likewise formed as areflective optical element and can be arranged outside the systems 102,104. Furthermore, the EUV radiation 108A can be directed onto thephotomask 120 via a mirror 122. As can be seen in FIG. 1A, the mirror122 is embodied as a grazing incidence mirror since the EUV radiation108A impinges with grazing incidence on a reflective surface of themirror. The photomask 120 has a structure which is imaged onto a wafer124 or the like in a reduced fashion via the projection system 104.

The projection system 104 (also referred to as projection lens) has sixmirrors M1 to M6 for imaging the photomask 120 onto the wafer 124. Inthis case, individual mirrors M1 to M6 of the projection system 104 canbe arranged symmetrically in relation to the optical axis 126 of theprojection system 104. It should be noted that the number of mirrors ofthe EUV lithography apparatus 100A is not restricted to the numberrepresented. A greater or lesser number of mirrors can also be provided.Furthermore, the mirrors are generally curved on their front face forbeam shaping. In another embodiment, the projection system 104 can beembodied without an optical axis, wherein one or more mirrors M1 to M6are embodied as freeform surfaces.

FIG. 1B shows a schematic view of the DUV lithography apparatus 100B,which includes a beam-shaping and illumination system 102 and aprojection system 104. DUV stands for “deep ultraviolet” and refers to awavelength of the working light of between 30 and 250 nm.

The DUV lithography apparatus 100B has a DUV light source 106B. By wayof example, an ArF excimer laser that emits radiation 108B in the DUVrange at 193 nm, for example, can be provided as the DUV light source106B.

The beam-shaping and illumination system 102 illustrated in FIG. 1Bguides the DUV radiation 108B onto a photomask 120. The photomask 120 isembodied as a transmissive optical element and can be arranged outsidethe systems 102, 104. The photomask 120 has a structure which is imagedonto a wafer 124 or the like in a reduced fashion via the projectionsystem 104.

The projection system 104 has a plurality of lens elements 128 and/ormirrors 130 for imaging the photomask 120 onto the wafer 124. In thiscase, individual lens elements 128 and/or mirrors 130 of the projectionsystem 104 can be arranged symmetrically in relation to the optical axis126 of the projection system 104. It should be noted that the number oflens elements and mirrors of the DUV lithography apparatus 100B is notrestricted to the number represented. A greater or lesser number of lenselements and/or mirrors can also be provided. Furthermore, the mirrorsare generally curved on their front face for beam shaping.

An air gap between the last lens element 128 and the wafer 124 can bereplaced by a liquid medium 132 which has a refractive index of greaterthan 1. The liquid medium can be high-purity water, for example. Such aconstruction is also referred to as immersion lithography and has anincreased photolithographic resolution.

FIG. 2A shows a first embodiment of a lithography system 200 having theprojection lens 104 and a wafer 124. The projection lens 104 includes acarrying frame 204, an optical element 208 (in the present case alsoreferred to as first optical element), a sensor subframe 212 (in thepresent case also referred to as first sensor subframe), a sensor 218(in the present case also referred to as first sensor) configured todetect a position of the optical element 208 with respect to the sensorsubframe 212, and a sensor 220 configured to detect a position of thewafer 124 with respect to the sensor subframe 212.

By way of example, the projection lens 104 includes only a single sensorsubframe 212. In this case, the sensor subframe 212 can be referred toas sensor frame 212. Furthermore, the sensor subframe 212 can bereferred to for example as sensor frame. The sensor subframe 212 iswholly or partly enclosed by the carrying frame 204. By way of example,the optical element 208 is linked or mounted with the aid of aconnecting or carrying structure 214A at the carrying frame 204, whereinthe optical element 208 is preferably mounted in such a way that in eachcase a controlled relative movement with respect to the carrying frame204 can be performed. The sensor subframe 212 is connected to thecarrying frame 204 in an oscillation-decoupled manner with the aid of amounting 214B.

The optical element 208 at least partly defines a beam path 224 of theprojection lens 104. The optical element 208 is arranged for example asthe last optical element in the beam path 224 upstream of the wafer 124.Thus a working light 226 is incident on the wafer 124 directly from theoptical element 208.

It goes without saying that a multiplicity of optical elements (notshown in FIG. 2A) can be referenced to the sensor subframe 212. By wayof example, the projection lens 104 includes a multiplicity of sensors(not shown in FIG. 2A) configured to detect positions of themultiplicity of optical elements relative to the sensor subframe 212. Byway of example, the projection lens 104 includes exactly 2, 3, 4, 5, 6,7, 8, 9, 10, 11, 12, 13 or 14 optical elements that are referenced tothe sensor subframe 212.

FIG. 2B shows a second embodiment of a lithography system 200 having theprojection lens 104 and a wafer 124. The projection lens 104 includesthe carrying frame 204, an optical element 206 (in the present case alsoreferred to as second optical element), the optical element 208 (in thepresent case also referred to as first optical element), a sensorsubframe 210 (in the present case also referred to as second sensorsubframe) and the sensor subframe 212 (in the present case also referredto as first sensor subframe). The optical element 206, the opticalelement 208, the sensor subframe 210 and the sensor subframe 212 can bearranged for example geometrically within the carrying frame 204. By wayof example, the optical elements 206, 208 are linked or mounted in eachcase with the aid of a connecting or carrying structure 214A at thecarrying frame 204, wherein the optical elements 206, 208 are preferablymounted in such a way that in each case a controlled relative movementwith respect to the carrying frame 204 can be performed. The sensorsubframes 210, 212 are connected to the carrying frame 204 in each casein an oscillation-decoupled manner with the aid of a mounting 214B.

Furthermore, the projection lens 104 includes a sensor 216 configured todetect a position of the optical element 206 with respect to the sensorsubframe 210. For an accurate position determination, the sensor 216 candetect for example three translational degrees of freedom and threerotational degrees of freedom of the optical element 206 with respect tothe sensor subframe 210. Consequently, a plurality of distances andangles between the measurement objects can be determined. In this case,an angle change can also be referred to as relative tilting. The sensor216 is arranged on the sensor subframe 210 and connected thereto,wherein, depending on the type of sensor, a part of the sensor 216 canalso be arranged on the optical element 206. Alternatively, the sensor216 can also just be arranged on the optical element 206 and connectedthereto. In other words, the optical element 206 is referenced to thesensor subframe 210 with the aid of the sensor 216.

Furthermore, the projection lens 104 includes the sensor 218 configuredto detect a position of the optical element 208 with respect to thesensor subframe 212. The statements in respect of the optical element206, the sensor subframe 210 and the sensor 216 are correspondinglyapplicable to the optical element 208, the sensor subframe 212 and thesensor 218.

Furthermore, the projection lens 104 includes the sensor 220 configuredto detect the position of the wafer 124 with respect to the sensorsubframe 212. The sensor 220 is arranged on the sensor subframe 212 andconnected thereto, wherein, depending on the type of sensor, a part ofthe sensor 220 can also be arranged on the wafer 124, wherein the wafer124 is arranged outside the carrying frame 204. Alternatively, thesensor 220 can also just be arranged on the wafer 124 and connectedthereto, wherein in this case the lithography system 200 can include thesensor 220 and not the projection lens 104. In other words, the wafer124 is referenced to the sensor subframe 212 with the aid of the sensor220. Thus the optical device 208 and the wafer 124 are referenced toexactly one sensor subframe 212. The sensor subframe 212 is embodied inparticular as a stiff and compact element.

Furthermore, the projection lens 104 moreover includes a sensor 222 (inthe present case also referred to as sixth sensor) configured to detecta position of the sensor subframe 210 with respect to the sensorsubframe 212. The description in respect of the sensor 216 iscorrespondingly applicable to the sensor 222.

Detecting a position of one element (e.g. optical element, wafer,reticle, sensor subframe, etc.) with respect to another element (e.g.optical element, wafer, reticle, sensor subframe, etc.) in the presentcase is also referred to as referencing (indicated by a circle and anarrow passing diagonally through the circle in FIG. 2A, 2B, 3 and 4).Even if not described or illustrated in the respective figure, onesensor or a plurality of sensors is or are assumed for the referencingor detection of a position.

The optical elements 206, 208 at least partly define the beam path 224of the projection lens 104. The optical element 208 is arranged as thelast optical element in the beam path 224 upstream of the wafer 124.Thus the working light 226 is incident on the wafer 124 directly fromthe optical element 208.

The optical element 208 has an LoS sensitivity and the wafer 124 has anLoS sensitivity. Optical elements, a reticle and/or wafers that arereferenced to a sensor subframe can be combined as a group, wherein ameasurement error of a sensor between this group and the remaining lens(remaining beam path 224 defining optical elements) acts for example ina manner like a sum of the LoS sensitivities of all the elements of thegroup. In the case of the projection lens 104 shown in FIG. 2A or 2B,for example the LoS sensitivity of the optical element 208 has adifferent sign from that of the LoS sensitivity of the wafer 124. Thishas the advantage that the LoS sensitivities almost completelycompensate for one another. By way of example, a sum formed from the LoSsensitivity of the optical element 208 and of the wafer 124 has anabsolute value of less than 0.5, less than 0.2, less than 0.1 or lessthan 0.01.

FIG. 3 shows a third embodiment of the lithography system 200. Incontrast to FIG. 2B, the projection lens 104 includes an optical element300 (in the present case also referred to as third optical element) thatfurther defines the beam path 224. Furthermore, the projection lens 104includes a sensor 302 (in the present case also referred to as fourthsensor) configured to detect a position of the optical element 300 withrespect to the sensor subframe 212. The description given above inrespect of the sensor 216 is correspondingly applicable to the sensor302 as well. Thus the optical element 208, the optical element 300 andthe wafer 124 are referenced to a single sensor subframe 212 from aplurality of oscillation-decoupled sensor subframes 210, 212.

The optical element 300 is arranged as the penultimate optical elementin the beam path 224. Thus the working light 226 is incident from theoptical element 300 directly on the optical element 208 and from theoptical element 208 directly on the wafer 124. Referencing thepenultimate optical element 300, the last optical element 208 and thewafer 124 to a single sensor subframe 212 has the advantage that LoSsensitivities of these elements at least for a translational shiftalmost completely compensate for one another.

The lithography system 200 includes the reticle 120, wherein theprojection lens 104 includes a sensor 306 (in the present case alsoreferred to as fifth sensor) configured to detect a position of thereticle 120 with respect to the sensor subframe 210. The sensor 306 isarranged on the sensor subframe 210 and connected thereto, wherein,depending on the type of sensor, a part of the sensor 306 can also bearranged on the reticle 120, wherein the reticle 120 is arranged outsidethe carrying frame 204. Alternatively, the sensor 306 can also just bearranged on the reticle 120 and connected thereto, wherein for examplein this case the lithography system 200 can include the sensor 306. Inother words, the reticle 120 is referenced to the sensor subframe 210with the aid of the sensor 306 and the optical element 206 is referencedto the sensor subframe 210 with the aid of the sensor 216. Thus thereticle 120 and the optical element 206 form a group in which themeasurement error of a sensor between this group and the remaining lenshas an effect with the sum of the LoS sensitivities (of the reticle 120and of the optical element 206). Preferably, an LoS sensitivity of thereticle 120 has a different sign from that of the LoS sensitivity of theoptical element 206, such that they mutually compensate for one anotherat least in part.

In addition, the projection lens 104 includes an actuator 308 (in thepresent case also referred to as first actuator) configured to positionthe optical element 206 with respect to the carrying frame 204.Furthermore, the projection lens 104 includes an actuator 310 configuredto position the optical element 208 with respect to the carrying frame204. Moreover, the projection lens 104 includes an actuator 312configured to position the optical element 300 with respect to thecarrying frame 204.

Furthermore, the projection lens 104 includes a control device 314,which is configured to drive the actuator 308 for positioning theoptical element 206 depending on the position detected by the sensor216, the position detected by the sensor 218 and the position detectedby the sensor 222. Furthermore, the control device 314 is configured todrive the actuator 312 for positioning the optical element 300 dependingon the position detected by the sensor 302 and the position detected bythe sensor 218. Furthermore, the control device 314 is configured todrive the actuator 310 for positioning the optical element 208 dependingon the position detected by the sensor 218. In other words, by way ofexample, all the optical elements 206, 300 arranged in the beam path 224follow the last optical element 208. Alternatively, the elementsarranged in the beam path 224 could follow another optical element orthe wafer 124. The communication of the sensors 216, 218, 220, 222, 302,306 and of the actuators 308, 310, 312 with the control device 314 isindicated by dashed lines in FIG. 3.

By way of example, a sum formed from the LoS sensitivity of the opticalelement 208, of the wafer 124 and of the optical element 300 has anabsolute value of less than 0.5, less than 0.2, less than 0.1 or lessthan 0.01.

The actuators 308, 310, 312 are embodied for example as Lorenzactuators. The optical elements 206, 208, 300 are embodied for exampleas mirrors M1-M6 and/or lens elements 128.

Although not illustrated, by way of example all the optical elementsfrom FIG. 1A, 1B, 2A, 2B and 4 can be actuated with the aid ofactuators. Correspondingly, a control device 314 is always present aswell. Furthermore, by way of example, corresponding sensors are alwaysprovided for referencing the optical elements.

FIG. 4 shows a fourth embodiment of the lithography system 200. Incontrast to FIG. 3, no sensors, no actuators, no beam path and nocontrol device are illustrated. Furthermore, besides the sensorsubframes 210, 212, the lithography system 200 includes a sensorsubframe 400 and a sensor subframe 402. The reticle 120 and a group 404(in the present case also referred to as first group) of opticalelements, including the optical element 206 and a further opticalelement, are referenced to the sensor subframe 210. The further opticalelement is arranged in the beam path 224 at the second location, suchthat working light 226 from the optical element 206 is incident directlyon the further optical element. A group 406 of optical elements isreferenced to the sensor subframe 400, wherein the group 406 includesoptical elements arranged at the third location and at the fourthlocation in the beam path 224. Furthermore, a group 408 of opticalelements is referenced to the sensor subframe 402, wherein the group 408includes optical elements arranged at the fifth location, the sixthlocation, the seventh location and the eighth location in the beam path224. As already shown in FIG. 3, the optical element 208, the opticalelement 300 and the wafer 124 are referenced to the sensor subframe 212.In this case, the optical element 300 is arranged at the ninth locationin the beam path 224 and the optical element 208 is arranged at thetenth location in the beam path 224. Consequently, the projection lens104 includes ten optical elements. By way of example, in each caseseparate linking structures 214A to the carrying frame 204 can beprovided for all the optical elements.

In contrast to FIG. 3, a position of the sensor subframe 210 withrespect to the sensor subframe 400, a position of the sensor subframe402 with respect to the sensor subframe 400 and a position of the sensorsubframe 212 with respect to the sensor subframe 400 are detected.

The carrying frame 204 is connected to a further carrying structure 410,in particular a base frame or a base, via a mounting 214B, wherein theconnection is embodied in particular as an oscillation-decoupledconnection.

Preferably, the projection lens 104 includes a mirror configurationincluding six normal incidence mirrors. Alternatively, four normalincidence mirrors and four grazing incidence mirrors 122 can be present.As a further alternative, the projection lens 104 can include threenormal incidence mirrors and seven grazing incidence mirrors 122. Theoptical elements 206, 208, 300 are selected from the mirrors of themirror configuration.

FIG. 5 shows a scheme for the position determination of the opticalelements. The mirrors M1-M6 from FIG. 1A were selected merely by way ofexample as optical elements, wherein the mirrors M1 and M2 arereferenced to the sensor subframe 210, the mirrors M3 and M4 arereferenced to the sensor subframe 400 and the mirrors M5 and M6 arereferenced to the sensor subframe 212.

For the actuation of the optical elements M1-M6, all the opticalelements are intended to follow the mirror M6, for example. Thedistances between the optical elements M1-M6 and the respective sensorsubframe 210, 212, 400 as considered in a spatial direction X aredesignated by X₁-X₆, wherein the distances are detected by correspondingsensors. A distance in the spatial direction X between the sensorsubframe 210 and the sensor subframe 400 is designated as S₁ and betweenthe sensor subframe 400 and sensor subframe 212 as S₂, whereincorresponding sensors detect the distances S₁, S₂. A distance S3 betweenthe sensor subframe 210 and sensor subframe 212 is not detecteddirectly, for example, but rather calculated by addition of thedistances S1 and S2, such that mention can be made of a virtual sensorbetween the sensor subframe 210 and the sensor subframe 212, forexample.

The alignment (actuation) of the mirrors M1 to M5 in the X-directioninvolves determining for example respective distances ΔX₅₆, ΔX₄₆, ΔX₃₆,ΔX₂₆ and ΔX₁₆ between the mirror M6 and the mirrors M1-M5. In this case,it holds true that:

ΔX ₅₆ =X ₅ −X ₆,

ΔX ₄₆ =S ₂ +X ₄ −X ₆,

ΔX ₃₆ =S ₂ +X ₃ −X ₆,

ΔX ₂₆ =S ₁ +S ₂ +X ₂ −X ₆, and

ΔX ₁₆ =S ₁ +S ₂ +X ₁ −X ₆.

Since the distances ΔX₅₆, ΔX₄₆, ΔX₃₆, ΔX₂₆ and ΔX₁₆ between the mirrorM6 and the mirrors M1-M5 are determined indirectly, mention can be madeof virtual sensors between the mirrors M1-M6. A measurement error occursfor each actual measurement of the distances X₁-X₆, S₁, S₂.

It has been recognized that the distances X1 to X6 must always bemeasured for corresponding systems and that the correspondingmeasurement errors (which occur during normal operation of a sensor)cannot be avoided. An incorrect position of the optical elements M1-M6on account of the measurement errors leads to a corresponding imagevibration since the measurement errors occur in particular randomly withrespect to one another, and, consequently, the mirrors M1-M6 have randomactual positions within a certain tolerance range. Moreover, it has beenrecognized that a reduction of measurements S₁, S₂ between the sensorsubframes 210, 400, 212 leads to a reduction of the measurement error.This is intrinsically a factor that opposes increasing and, undercertain circumstances, supports reducing the number of sensor subframes.Nevertheless, it should be taken into consideration that a division ofthe sensor frame into a multiplicity of sensor subframes is advantageousfor reducing the quasi-static deformation.

Furthermore, it has been recognized that measurement errors which occurduring the measurement of the distances S₁, S₂ have a small influence onthe image effect and thus on the image vibration when the sum of the LoSsensitivities of the optical elements M1, M2, M3, M4 that are referencedto a respective sensor subframe 210, 410 is as small as possible. Thisstems from the fact that the same measurement error that results fromthe distance measurement of the distance S₂ influences a respectivepositioning manipulated variable of the mirror M3 and of the mirror M4.In other words, the mirror M3 and the mirror M4 have incorrect positions(considered in isolation for the measurement error resulting from themeasurement of the distance S₂) which have the same direction and thesame absolute value, such that the incorrect positions compensate forone another in the image effect on account of compensating LoSsensitivities of the mirrors M3 and M4. Analogously thereto, themeasurement errors from the distance measurements of the distances S₁,S₂ (as S₃) simultaneously influence the respective positioningmanipulated variable of the mirror M1 and of the mirror M2. Furthermore,it is conceivable to provide such sensors for the position detection ofthe optical elements relative to a sensor subframe which simultaneouslycause measurement errors which have an identical sign and a similarabsolute value, such that a compensating effect in the image effectoccurs when the measurement errors influence the positioning manipulatedvariables of the mirrors.

Consequently, via a skilful arrangement of the referencings of theoptical elements M1-M6 to the sensor subframes 210, 400, 212, it ispossible to reduce the individual sums of the LoS sensitivities, inparticular by grouping optical elements having LoS sensitivities havingdifferent signs and, if appropriate, having absolute values of similarmagnitude (e.g. in the case of exactly two optical elements). Since achange in the number of sensor subframes or a change in the referencingof an optical element has system- or design-dictated restrictions, thesemust correspondingly influence a selection process.

FIG. 6 shows a block diagram of a method for producing the lithographysystem 200, in particular the projection lens 104.

A step S1 involves arranging the optical elements 206, 208 for definingthe beam path 224. By way of example, 6, 8 or 10 optical elements, inparticular mirrors, can also be arranged.

A step S2 involves providing the wafer 124 at the end of the beam path224 and/or the optical element 300 for further defining the beam path224. By way of example, step S2 can additionally involve providing thereticle 120.

A step S3 involves determining the LoS sensitivities of the opticalelements 206, 208 and also of the wafer 124 and/or of the opticalelement 300. Determining the LoS sensitivities is carried out separatelyin particular for a constant numerical aperture and in particular forshifts in three spatial directions and for tilts about three axes. Byway of example, the LoS sensitivities of all the elements (reticle,wafer, all optical elements provided) are determined. Since it isadvantageous for LoS sensitivities of the elements (in particular ofreticle, wafer, and optical elements) to have different signs, steps S1and S2 can be carried out once again or influenced until the determinedLoS sensitivities of the elements have different signs. In other words,at least two elements which have LoS sensitivities having differentsigns should be grouped.

A step S4 involves assigning the LoS sensitivities to the sensorsubframe 210 and sensor subframe 212 in accordance with a first of Ncombination possibilities. The combination possibilities which arepossible purely computationally can be reduced for example by system- ordesign-dictated restrictions. A table 600 from FIG. 6 visualizes forexample three of N combination possibilities for two sensor subframes210, 212 and three optical elements 206, 208, 300. Referencings 602 ofthe optical elements 206, 208, 300 are represented here in each casewith the aid of a connecting line.

A step S5 involves summing the LoS sensitivities assigned to arespective sensor subframe 210, 212, provided that not just oneindividual LoS sensitivity is assigned. This summing is carried outseparately for the LoS sensitivities of the different shifts and tilts.

By way of example, this can be carried out just for one representativeshift or tilt or for shifts in three spatial directions and tilts aboutthree axes. A table 604 from FIG. 6 illustrates by way of example asummation for three of N combination possibilities for two sensorsubframes 210, 212 and three optical elements 206, 208, 300. The LoSsensitivities of the three optical elements 206, 208, 300 are designatedby way of example by a₁-a₃. A sum S₂₁₀ of the LoS sensitivities of theelements (in particular reticle, wafer, optical elements) which arereferenced to the sensor subframe 210 is formed. Furthermore, a sum S₂₁₂of the LoS sensitivities of the elements (in particular reticle, wafer,optical elements) that are referenced to the sensor subframe 212 isformed. If further sensor subframes 400, 402 are present, in each casesums S₄₀₀, S₄₀₂ (not shown in FIG. 6) of the LoS sensitivities of theelements (reticle, wafer, optical elements) that are referenced to thefurther sensor subframes 400, 402 are correspondingly formed as well. Inparticular, 2 to 6 dimensional vectors for the LoS sensitivities canalso be present, which are correspondingly summed.

A step S6 involves storing the sums S₂₁₀, S₂₁₂, S₄₀₀, S₄₀₂ andindividual LoS sensitivities S₂₁₀, S₂₁₂, S₄₀₀, S₄₀₂. This is done forexample for a later comparison.

A step S7 involves repeating steps S4-S6 up to and including the Nthcombination possibility.

A step S8 involves selecting one combination possibility from the Ncombination possibilities depending on the stored sums S₂₁₀, S₂₁₂, S₄₀₀,S₄₀₂ and individual LoS sensitivities S₂₁₀, S₂₁₂, S₄₀₀, S₄₀₂. By way ofexample, absolute values of the sums S₂₁₀, S₂₁₂, S₄₀₀, S₄₀₂ andindividual LoS sensitivities S₂₁₀, S₂₁₂, S₄₀₀, S₄₀₂ of a respectivecombination possibility can be formed and subsequently be summed inorder to form a total sum. Alternatively, the absolute values formed canbe weighted with a respective factor before the step of forming thetotal sum in order to take account of system- or design-dictatedboundary conditions or restrictions, for example. Correspondingly,absolute values of sum vectors S₂₁₀, S₂₁₂, S₄₀₀, S₄₀₂ can also be formedand, if appropriate, weighted with a factor. By way of example, thatcombination possibility from the N combination possibilities which hasthe smallest total sum can subsequently be selected.

A step S9 involves providing sensors 216, 218, 220, 302, 306 configuredto detect a respective position of the optical elements 206, 208 andalso of the wafer 124 and/or of the optical element 300 with respect toa reference.

A step S10 involves selecting the sensor subframe 210 or sensor subframe212 as reference depending on the selected combination possibility. Iffurther sensor subframes 400, 402 are present, they are alsocorrespondingly taken into account concomitantly in the selectionprocess.

Individual or a plurality of steps S1-S10 can be performed in asoftware-based simulation model.

Although the present disclosure has been described on the basis ofexemplary embodiments, it is modifiable in diverse ways.

LIST OF REFERENCE SIGNS

-   100A EUV lithography apparatus-   100B DUV lithography apparatus-   102 Beam-shaping and illumination system-   104 Projection system, projection lens,-   106A EUV light source-   106B DUV light source-   108A EUV radiation-   108B DUV radiation-   110-118 Mirrors-   120 Photomask, reticle-   122 Mirror, grazing incidence mirror-   124 Wafer-   126 Optical axis-   128 Lens element-   130 Mirror-   132 Immersion liquid-   200 Lithography system-   204 Carrying frame-   206 Optical element-   208 Optical element-   210 Sensor subframe-   212 Sensor subframe-   214A Connecting structure, carrying structure-   214B Mounting-   216 Sensor-   218 Sensor-   220 Sensor-   222 Sensor-   224 Beam path-   226 Working light-   300 Optical element-   302 Sensor-   306 Sensor-   308 Actuator-   310 Actuator-   312 Actuator-   314 Control device-   400 Sensor subframe-   402 Sensor subframe-   404 Group-   406 Group-   408 Group-   410 Carrying structure, base frame-   600 Table-   602 Referencing-   604 Table-   N Number of combination possibilities-   S₂₁₀ Sum-   S₂₁₂ Sum-   S₄₀₀ Sum-   S₄₀₂ Sum-   X Spatial direction-   ΔX₁₆ Distance-   ΔX₂₆ Distance-   ΔX₃₆ Distance-   ΔX₄₆ Distance-   ΔX₅₆ Distance-   a₁-a₃ LoS sensitivities-   M1-M6 Mirrors-   S1-S10 Method steps-   S₁-S₃ Distance-   X₁-X₆ Distance

What is claimed is:
 1. A lithography system, comprising: a projection lens, comprising: a first optical element; a first sensor subframe; a first sensor which is configured to detect a position of the first optical element with respect to the first sensor subframe; and a second sensor which is configured to detect a position of a wafer with respect to the first sensor subframe.
 2. The lithography system of claim 1, wherein: the projection lens further comprises a second optical element; the first and second optical elements at least partly define a beam path of the projection lens; and the first optical element is the last optical element in the beam path upstream of the wafer.
 3. The lithography system of claim 1, wherein the projection lens further comprises: a second optical element; a second sensor subframe which is oscillation-decoupled from the first sensor subframe; and a third sensor which is configured to detect a position of the second optical element with respect to the second sensor subframe.
 4. The lithography system of claim 3, wherein: the first and second optical elements at least partly define a beam path of the projection lens; and the first optical element is the last optical element in the beam path upstream of the wafer.
 5. The lithography system of claim 3, further comprising the wafer.
 6. The lithography system of claim 3, wherein: the projection lens further comprises a third optical element and a fourth sensor; the first, second and third optical element at least partly define a beam path of the projection lens; the fourth sensor is configured to detect a position of the third optical element with respect to the first sensor subframe; and the third optical element is the penultimate optical element in the beam path.
 7. The lithography system of claim 6, wherein the first optical element is the last optical element in the beam path upstream of the wafer.
 8. The lithography system of claim 7, wherein the projection lens further comprises a fifth sensor which is configured to detect a position of the reticle with respect to the second sensor subframe.
 9. The lithography system of claim 8, wherein the projection lens further comprises a sixth sensor which is configured to detect a position of the second sensor subframe with respect to the first sensor subframe.
 10. The lithography system of claim 6, wherein the projection lens further comprises a fifth sensor which is configured to detect a position of the reticle with respect to the second sensor subframe.
 11. The lithography system of claim 10, further comprising a reticle.
 12. The lithography system of claim 10, wherein the projection lens further comprises a sixth sensor which is configured to detect a position of the second sensor subframe with respect to the first sensor subframe.
 13. The lithography system of claim 12, wherein the projection lens further comprises: a carrying frame; a first actuator which is configured to position the second optical element with respect to the carrying frame; a second actuator which is configured to position the first optical element with respect to the carrying frame; and a control device which is configured to drive the first actuator to position the second optical element depending on: i) the position detected by the third sensor; ii) the position detected by the first sensor; and iii) the position detected by the sixth sensor; and the control device is configured to drive the second actuator to position the first optical element depending on the position detected by the first sensor.
 14. The lithography system of claim 3, wherein: the first optical element comprises a mirror or a lens; the second optical element comprises a mirror or a lens; and the third optical element comprises a mirror or a lens.
 15. The lithography system of claim 3, wherein: the first optical element comprises a mirror; the second optical element comprises a mirror; and the third optical element comprises a mirror.
 16. The lithography system of claim 13, wherein: the projection lens comprises a mirror configuration which comprises the first, second and third mirrors; and at least one of the following holds: the mirror configuration comprises six normal incidence mirrors; the mirror configuration comprises four normal incidence mirrors and four grazing incidence mirrors; and the mirror configuration comprises three normal incidence mirrors and seven grazing incidence mirrors.
 17. The lithography system of claim 1, wherein: the first optical element has a line-of-sight sensitivity having a first sign; the wafer has an line-of-sight sensitivity having a second sign; and the first sign is a different from the second sign from.
 18. The lithography system of claim 17, wherein a sum of a line-of-sight sensitivity of at least one member selected from the group consisting of the first optical element, the wafer and the third optical element has an absolute value of less than 0.5.
 19. A method, comprising: a) arranging first and second optical element to define a beam path; b) providing at least one member selected from the group consisting of a wafer at the end of the beam path and a third optical element for further defining the beam path; c) determining the line-of-sight sensitivities of the first and second optical elements and also of the wafer and/or of the third optical element; d) assigning the line-of-sight sensitivities to a first and second sensor subframe in accordance with a first of N combination possibilities; e) summing the line-of-sight sensitivities assigned to a respective sensor subframe, provided that not just one individual line-of-sight sensitivity; f) storing the sums and individual line-of-sight sensitivities; g) repeating d)-f) up to and including the Nth combination possibility; h) selecting a combination possibility from the N combination possibilities depending on the stored sums and individual line-of-sight sensitivities; i) providing sensors to detect a respective position of the first and second optical element and also of the wafer and/or third optical element with respect to a reference; and j) selecting the first or second sensor subframe as reference depending on the selected combination possibility.
 20. The method of claim 19, wherein the line-of-sight sensitivities determined in c) have different signs. 